Issue with ribbed surface (C-axis milling)

So I've encountered this problem, which neither I or the machine technician are able to explain.

I operate a Mori Seiki MT2000-SZ (similar to the NTX2000) and during XY-milling with the upper turret we get four backlash lines. But when we C-axis mill the entire surface gets covered in lines, initially we only had this problem with the upper turret, but now we've encountered the same problem with the lower turret two (which only has X, Z and C).

We've made this piece before (half a year ago or so and we didn't have this problem), now the machine might have taken a blow since then so it's fully possible that things aren't alligned properly.

The finish cut is done with simple G1s, G2s and G3s so it's not like it's a patchwork of a bunch of small coordinates.